1. Field of the Invention
The present invention relates to thermally developable light-sensitive materials and, more particularly, it is concerned with thermally developable light-sensitive materials containing a novel, improved color toning agent.
2. Description of the Prior Art
It is known that photographic materials containing a light-sensitive material such as silver halides can be used to produce images by heating, that is to say, using the socalled dry processing. The most promising light-sensitive materials that can be used to produce photographic inages using such a dry processing at the present time are thermally developable light-sensitive materials which utilize compositions capable of forming images in which an oxidation-reduction image forming composition comprising, as essential components, organic silver salt oxidizing agents (such as silver behenate), reducing agents and a small amount of a photocatalyst such as a light-sensitive silver halide, is utilized, e.g., as disclosed in U.S. Pat. Nos. 3,152,904, 3,457,075, 3,707,377, 3,909,271 and so on. Such light-sensitive materials are stable at ordinary temperatures, but when heated to a temperature higher than about 80.degree. C. and preferably, higher than 100.degree. C., after exposure to light through an original image, silver is produced by an oxidation-reduction reaction between the organic silver salt oxidizing agent and the reducing agent present in the light-sensitive layer due to the catalytic action of the exposed photocatalyst present in proximity thereto whereby the exposed areas of the light-sensitive layer rapidly blacken and a contrast between the blacked areas and unexposed areas (background) is produced. Thus, images are formed.
In this light-sensitive system, photocatalysts in the light-sensitive materials after the completion of development are allowed to remain without stabilization to light, even though they change color upon exposure to light. Nevertheless, this system can produce satisfactory results, just as in the case where the residual photocatalyst receives a certain stabilizing treatment for light. This is because only a small amount of photocatalyst is employed in the system and a large portion of the silver component is present in a form of white or slightly colored organic silver salts which blacken upon light-exposure with difficulty and, therefore, even if color change occurs due to the decomposition of a small amount of residual photocatalyst through exposure to light, such a slight color change is difficult to discern visually.
In many cases, yellowish-brown colored images are produced with the above-described thermally developable light-sensitive materials utilizing silver salt oxidizing agents. However, the addition of color toning agents thereto can improve the color tone and achieve a preferred black color tone. Examples of such color toning agents include phthalazinones, oxazinediones, cyclic imides, urazoles, imino compounds such as 2-pyrazoline-5-ones and mercapto compounds as disclosed in, for example, U.S. Pat. Nos. 3,846,136, 3,782,941, 3,844,797, 3,832,186, 3,881,938 and 3,885,967, British Patent No. 1,380,795, Japanese Patent Application (OPI) Nos. 151138/75, 91215/74, 67132/75, 67641/75, 114217/75, 32927/75 and 22431/76, Japanese Patent Application No. 16128/76 and so on. However, these known color toning agents have some weak points. For instance, phthalazinones which exhibit the best color toning effect have a marked tendency to sublime and some of them quickly lose their color toning ability with the lapse of time under high humidity conditions, although desired color toning effects immediately after the preparation are achieved. The sublimation inherent in phthalazinones is disadvantageous since the inside of the developing device is contaminated by the sublimed phthalazinones adhering to and accumulating on the inside walls of the device used. Further, phthalazinones generate an unpleasant smell which permeates the area during the developing process. The introduction of substituents into the condensed benzene nucleus of phthalazinone has been attempted with the intention of suppressing the subliming ability of phthalazinone, but another new disadvantage of slow thermal development occurs with the resulting phthalazinone derivatives. Therefore, a satisfactory solution of the above-described problem has not yet been achieved.
Also, oxazinediones (e.g., benzoxazinediones as disclosed in U.S. Pat. No. 3,855,967) and cyclic imides (that is, phthalimides, 2,4-thiazolidinediones, 4-cyclohexane-1,2-dicarboxyimides and glutarimides as disclosed in U.S. Pat. No. 3,846,136), which are other classes of typical color toning agents, only cause a color toning effect upon certain specific organic silver salts, and possess such an insufficient color toning effect, compared with that of phthalazinone, that only brown or yellow colored images are achieved even immediately after the preparation of thermally developable light-sensitive materials of the above-described kind.